The present invention relates to vibration testing, and more particularly to a vibration testing apparatus and method using generation of acoustical waves as a means for imparting vibration to an article under test.
Many types of testing equipment have been developed over the past years to subject articles to vibration for the purpose of assessing their reliability, generally according to industry standards considering the ultimate conditions of use of the specific articles to be tested. In the past years, military, aerospace and other electronic technology-related industries have developed methods, guidelines and standards involving a stimulation technique known as Environmental Stress Screening (ESS) which aims at precipitating latent defects before the delivery of electronic and/or electromechanical products, which defects would otherwise be likely to reveal only where the products are in the field, thereby causing unpredictable product failures, generally at an early stage of the product life. ESS involves performing series of testing steps integrated into the manufacturing process of a product, which steps consist of subjecting the product to predetermined stress levels, depending upon the manufacturing techniques used, in order to detect defects during the manufacturing process. Such defects normally cannot be detected by visual inspection or conventional qualification and/or reliability testing carried out at the end of the manufacturing process. Effectiveness of ESS is mainly due to the fact that the stress (amplitude and duration) required to reveal a latent defect is generally not sufficient to cause a damage that could adversely affect the life of a faultless product.
While improving quality and reliability of products which become therefore more competitive, ESS may significantly reduce production, maintenance and return costs caused by product failures. While in the early years of ESS, testing procedures were limited to static thermal cycles and sinusoidal vibration cycles, testing procedures have been thereafter improved to include dynamic thermal cycles and random vibration cycles. ESS applications for the U.S. navy have been documented in the  less than  less than Navy Manufacturing Screening Program  greater than  greater than  NAVMAT-9492,1979, and thereafter, U.S. army has been using ESS to ensure a very high reliability as required for critical and complex electronic systems, and military standards such as  less than  less than Environmental Stress Screening Process for Electronic Equipment)  greater than  greater than  MIL-HDBK-2164A have been developed. More recently, ESS testing has spread to many fields of the civil industry to improve the quality of electronic and electromechanical commercial products while reducing manufacturing costs. ESS guidelines for testing those commercial products have been published, such as  less than  less than Environmental Stress Screening Guidelines for Assemblies  greater than  greater than , Institute of Environmental Sciences, March 1990, and  less than  less than Product Reliability Division Recommended Practice 001.1, Management and Technical Guidelines for the ESS Process  greater than  greater than  Institute of Environmental Sciences and Technology, January 1999. According to NAVMAT-9492 and as shown by the Power Spectral Density (PSD) reference curve 10 of FIG. 1, ESS vibration testing equipment must produce vibrations within the 20 to 2000 Hz frequency range during about 10 minutes, with a nominal vibration (acceleration) level of near 0.04 g2/Hz, which corresponds to an effective level of 6 grms as obtained by integrating the NAVMAT PSD reference curve. The NAVMAT-9492 guidelines, which were not published as an actual standard, cannot be considered as being appropriate to every products. Indeed, for certain types of electronic products, its use may be adversely damaging. In other cases, stress levels higher than the NAVMAT-9492 guidelines should be used, as for the recent ESS 2000 Project according to which the use of a nominal vibration (acceleration) level up to 20 grms is contemplated. Since each electronic circuit is provided with specific dynamic characteristics, the vibrating response of the circuit not only depends on the nature of the excitation, but also on the specific dynamic characteristics.
For the purpose of performing ESS vibrating stimulation, an electrodynamic vibration table is generally used since it provides adequate control over the vibration parameters to comply with ESS specifications. However, the high cost of such equipment may significantly reduce the economical benefit obtained from ESS when the volume of production is not sufficient, limiting the use of electrodynamic vibration tables for ESS testing to large manufacturing facilities. While other technological solutions or less expensive vibrating equipment is available, such as hydraulic or pneumatic vibration tables, those vibrators are generally not suitable for producing ESS stimulation. The use of hydraulic vibrators being limited to low vibration frequencies, the upper portion of the frequency spectrum of a typical ESS power density profile cannot be handled. While pneumatic vibrators can handle higher vibration frequencies, they generally cannot allow accurate control over the excitation signal which is required by ESS to provide a stimulation profile adapted to a specific product, as discussed in xe2x80x9cImproper Environmental Stress Screening Can Damage Your Productxe2x80x9d, Howe E., Test Engineering and Management, October/November 1998, pp. 22-23, and in xe2x80x9cImproper Environmental Stress Screening Can Damage Your Productxe2x80x94Part IIxe2x80x9d, Howe E., Test Engineering and Management, December/January 1998-99, pp. 14-16. In some cases, variation of the amplitude level may reach more than 30 dB.
Over the past years, acoustic test chambers have been developed to carry out various acoustical vibration tests, in which an article to be tested, such as an aircraft part, is subjected to a high sound pressure level which imparts vibration thereto. Such prior art chambers are disclosed in U.S. Pat. No. 3,104,543, U.S. Pat. No. 3,198,007, U.S. Pat. No. 3,827,288 and U.S. Pat. No. 4,574,632. More recently, in U.S. Pat. No. 5,226,326 issued to Polen et al., it was proposed to use a vibration chamber provided with a pair of speakers characterized by a same frequency operating range and arranged in a push-pull configuration to impart multiple modes of random vibration on a article under test according to a ESS vibration profile that is characterized by an enhanced power density level as compared with the profile obtained from a conventional in-phase arrangement of speakers. The acoustical waves providing direct vibrating stimulation to the product, the acoustic chamber does not require the use of custom mechanical fixtures which are specific to each product to be tested, so that universal-type fixtures can be generally used. While being less expensive than electrodynamic equipment, such a prior art acoustic test chamber may not provide the accurate control over a specific portion of the frequency spectrum which is required for testing particular products, since the proposed puss-pull configuration of identical speakers provides an overall increase of power spectral density essentially over the whole frequency range of the profile. Accurate control is particularly important within the lower frequency range of the profile where optimal stimulation of the main vibration modes is critical, as shown by the typical experimental Power Spectral Density curve designated at numeral 12 in FIG. 1, where a prior art acoustical testing chamber was used to test a plain printed circuit board without components mounted thereon, with white noise excitation signal characterized by a with a 500 Hz crossover frequency. It can be seen from FIG. 1 that the frequency response in the lower part of the operating bandwidth is significantly lower than PSD reference curve 10 guideline, indicating that the level of stress effectively applied to the article under test is insufficient. Although a nominal vibration (acceleration) level of about 14.5 grms can be estimated, which is well beyond the NAVMAT guidelines, integration of negative and positive variations exhibited by experimental curve 12 as compared to reference curve 10 allows an estimation of total negative and positive variations of about respectively 4.3 grms and 13.9 grms, which positive variation is essentially associated with the upper portion of the frequency range over about 1000 Hz. Although the positive variation could be better controlled by varying the amplitude characteristic of the excitation signal at high frequency to attenuate the positive variation at a desired level, experiences have shown that in the low frequency range, there is a limit in the excitation signal amplitude over which the negative variation cannot be significantly further reduced, thereby limiting the reliability and effectiveness of the ESS technique.
Accurate control is also important in the area of the crossover frequency when a crossover device is used to drive speakers having different operating ranges. In the latter case, accurate control of the power spectral density profile in the area of the crossover frequency should be obtained neither at the expense of power spectral efficiency, nor to adversely increase the overall acoustic level in the testing chamber so as to contribute to a more comfortable working environment.
It is therefore an object of the present invention to provide a testing apparatus and method for imparting vibration to an article under test, which provide improved control over the power spectral density profile of the imparted vibration.
It is a further object of the present invention to provide a testing apparatus and method for imparting vibration to an article, which can be used for the purpose of Environmental Stress Screening procedures.
It is a still further object of the present invention to provide a testing apparatus and method for imparting vibration to an article, which can be simultaneously subjected to thermal cycling.
According to the above objects, from a further broad aspect of the present invention, there is provided a vibration testing apparatus comprising a main enclosure defining a main acoustical cavity and having a baffle provided with at least one main opening, and an acoustical source having at least one acoustical transducer being acoustically coupled to the main acoustical cavity to generate low frequency acoustical waves toward the opening. The apparatus is capable of receiving an article to be tested in a position where the main opening is substantially closed by the article to expose a surface thereof to said acoustical wave while attenuating-portion of the acoustical waves reaching a substantially opposed surface of said article which is not directly exposed to the acoustical waves. Conveniently, the vibration testing apparatus further comprises acoustical insulation means adapted to receive the article an attachment means for securing the article in testing position, wherein the main opening is further closed by the acoustical insulation means.
According to another aspect of the invention, the vibration testing apparatus further comprises a thermally insulated enclosure defining a thermal cavity within which the article is contained; means for generating a flow of inert gas; means for heating said flow of inert gas; means for cooling said flow of inert gas; means for circulating said flow of inert gas into said thermal cavity; first sensor means located within said thermal cavity for producing a first temperature indicative signal; controller means responsive to said temperature indicative signal and operatively coupled to said heating means and said cooling means for controlling the temperature of one of said inert gas and said article by selectively activating one of said heating means and said cooling means according to a predetermined thermal cycling profile while said acoustical source generates said acoustical waves toward the exposed surface of said article.
From a still further broad aspect of the present invention, there is provided a vibration testing method comprising the steps of: a) providing a main enclosure defining a main acoustical cavity and having a baffle provided with at least one main opening; b) generating acoustical waves within a low frequency spectrum toward the opening; c) disposing at least one article to be tested in a position where the main opening is substantially closed by the article to expose a surface thereof to said acoustical waves of low frequency range while attenuating portion of the acoustical waves reaching a substantially opposed surface of the article which is not directly exposed to the low frequency acoustical waves. Conveniently, the method further comprises a step of: d) generating acoustical waves within a frequency spectrum higher to said low frequency spectrum toward the opposed article surface, said low and higher frequency spectrums being complementary adjacent a crossover frequency and being substantially in opposed phase relationship in the area of the crossover frequency to further increase power efficiency in said frequency area.